1. Field of the Invention
The present invention relates to DC voltage conversion circuits, and in particular, to DC—DC voltage conversion circuits.
2. Description of the Related Art
As is well known, a DC-to-DC converter is a circuit that converts a DC input voltage to a DC output voltage, with the output voltage typically being different, i.e., having a different magnitude (irrespective of polarity), than the input voltage. There are at least three general types: a step down converter, also known as a “buck” converter; a step up converter, also known as a “boost” converter; and a variable converter capable of producing an output voltage which is selectively a fraction or multiple of the input voltage, also known as a “buck-boost” converter.
Referring to FIG. 1A, a conventional buck converter accepts a DC input voltage VIN to produce a DC output voltage VOUT. The input voltage VIN is applied across a shunt rectifier in the form of a diode D in a switched manner by virtue of a series switch in the form of a transistor Q which is turned on and off according to the asserted and de-asserted states, respectively of a control voltage VC. (It should be noted that throughout this discussion, the transistors used as the switching elements are depicted as NPN bipolar junction transistors. However, it will be readily understood by one of ordinary skill in the art that, in accordance with well known circuit design principles, other transistors, such as PNP bipolar junction transistors and metal oxide semiconductor field effect (MOSFET) transistors, both N-type and P-type, can be substituted with appropriate reversals in voltage polarities as necessary.)
When the transistor Q is turned on, current will begin to flow through the inductor L to the shunt output capacitor C, and thereby begin charging the capacitor C. When the transistor Q is turned off, the inductor current will continue to flow, but now through the diode D instead of the transistor Q. As the control voltage VC is periodically asserted and de-asserted, this process repeats, thereby producing a DC output voltage VOUT with an average value that is proportional to the input voltage VIN, with such proportion, or fraction, being approximately equal to the duty cycle (ratio of the asserted state duration to the sum of the asserted and de-asserted states durations) of the control voltage VC.
Referring to FIB. 1B, a conventional boost converter also has the inductor L in series (now at the input), but now has the diode D in series and the switching transistor Q connected in shunt, substantially as shown. During assertion of the control voltage VC, the transistor Q is turned on, thereby initiating current flow through the inductor L. During de-assertion of the control voltage VC, the transistor Q is turned off, and the inductor current flows through the diode D to charge the capacitor C. This produces an output voltage VOUT which is a multiple of the input voltage VIN, with such multiple being approximately equal to the inverse of the difference between unity and the duty cycle of the control voltage VC.
Referring to FIG. 1C, a conventional buck-boost converter has the transistor Q in series, like the buck converter, and the diode D in series (but reversed in polarity) like the boost converter. The inductor L is now connected in shunt between the transistor Q and diode D substantially as shown. During assertion of the control voltage VC, the transistor Q is turned on and inductor current flows. During de-assertion of the control voltage VC, the transistor Q is turned off, thereby causing the continuing inductor current to flow through the diode D and charge the capacitor C. This produces a DC output voltage VOUT with an average value that is selectively a fraction or multiple of the input voltage VIN. Such fraction or multiple is approximately equal to the quotient of the duty cycle (of the control voltage VC) divided by the difference between unity and the duty cycle. Hence, for duty cycles less than 0.5 this circuit is a buck converter, while for duty cycles greater than 0.5 this circuit is a boost converter.
As is well known, these types of DC—DC converters have historically been implemented using discrete components, primarily due to the amount of current required to maintain the average output voltage with as little voltage ripple as possible. As a result, in order to also maintain some minimum efficiency on part of the inductor, the size of the inductor is often larger than what might be desired for a more compact circuit design, since reducing the size of the inductor will increase the likelihood of magnetic saturation, thereby decreasing the efficiency.